sortedcache: close the fd on stat trouble as well

As Coverity pointed out, we fail to close the file handle if we have
trouble during the stat call. This is unlikely to result in real leaks
as stat errors should be very rare, but do make sure to close the fd.
diff --git a/src/sortedcache.c b/src/sortedcache.c
index ed4199b..5bd989a 100644
--- a/src/sortedcache.c
+++ b/src/sortedcache.c
@@ -216,6 +216,7 @@
 	if (p_fstat(fd, &st) < 0) {
 		giterr_set(GITERR_OS, "failed to stat file");
 		error = -1;
+		(void)p_close(fd);
 		goto unlock;
 	}